A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus, a position sensor is used to measure the position of movable objects, such as the substrate table and the patterning device support. This position is measured with high accuracy and usually in multiple degrees of freedom.
A position measurement system used in a stage system, such as a lithographic apparatus stage system is an encoder-type sensor including an encoder head and a grid plate. The encoder head includes a light source to emit light towards a measurement location on the grid plate and a number of detectors to receive the light reflected by the grid plate. On the basis of phase differences between the light received by the two or more detectors, the position of the movable object with respect to the grid plate may be determined.
Such encoder-type position measurement system may provide a position of a movable object with high accuracy and in multiple degrees of freedom. One encoder head may be capable of determining a position in one or more degrees of freedom. The encoder system is a two dimensional encoder measurement system wherein the encoder head can measure its position with respect to the grid plate in both a direction parallel to the grid plate and a direction perpendicular to the direction of the grid plate. Three of such encoder heads provide a system which can measure the position of the movable object with respect to the grid plate in six degrees of freedom.
Due to a manufacturing process of the grid plate, the grid or grating may include a defect. In particular, the reflectivity of the grid plate may deviate at particular location(s) due to such defect. Such defects may lead to inaccurate position measurements and are therefore undesired. To avoid inaccurate position measurements, the requirements during a production process of a grid plate are very high, and only a limited number of produced grid plate have sufficient quality to be used in a lithographic apparatus. As a result, the costs for grid plates are considerable high.
Furthermore, during use, defects may come into existence, for instance due to contamination, cleaning or physical damage. Also these defects may lead to inaccurate position measurement of a movable object for which it is used.